Input buffer with automatic switching point adjustment circuitry, and synchronous DRAM device including same

ABSTRACT

An input buffer is presented for buffering an input signal having a voltage magnitude which alternates between a first voltage level and a second voltage level, where the first and second voltage levels may vary over time. In one embodiment, the input buffer includes a first and second detector circuits, an average generator circuit, and a differential amplifier. The first detector circuit receives the input signal and produces a first signal having a magnitude indicative of the first voltage level. The second detector circuit receives the input signal and produces a second signal having a magnitude indicative of the second voltage level. The average generator circuit receives the first and second signals, and uses the magnitudes of the first and second signals to produce a third signal having a magnitude indicative of a third voltage level substantially mid way between the first voltage level and the second voltage level. The third voltage level defines a variable an automatically adjusted “switching point”. The differential amplifier receives the input signal, the third signal, and a first and second power supply voltages. The differential amplifier amplifies a difference between the voltage magnitude of the input signal and the third voltage level in order to produce an output signal which alternates between the first and second power supply voltages. An integrated circuit is described including the input buffer coupled between one of a set of input/output pads and circuitry, wherein the circuitry may be synchronous dynamic random access memory (SDRAM) circuitry.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to digital logic circuits, and, moreparticularly, to digital logic circuits which receive signalsalternating between two different voltage levels.

2. Description of the Related Art

Digital logic circuits typically quantize signal voltage levels torepresent two or more logic levels or states. In positive binary logic,a signal voltage level within a voltage range extending downward from amaximum operating voltage typically represents a logic “high” level(e.g., a logic ‘1’ state), and a signal voltage level within a voltagerange extending upward from a minimum operating voltage typicallyrepresents a logic “low” level (e.g., a logic ‘0’ state).

Electromagnetic events occurring in and around digital logic circuitsproduce unwanted “noise” signals. These unwanted noise signals may becoupled into nodes or signal lines of digital logic circuits, causingvoltage level variations of logic signals on the nodes or signal lines.For this reason, reliable digital logic circuits incorporate “noisemargins” (NM) for both the logic high level (NM_(H)) and the logic lowlevel (NM_(L)). When a noise signal has an amplitude less than NM_(L),and a logic gate receives a signal at an input representing the logiclow level and including the noise signal, the noise signal will beattenuated as the received signal passes from the input to an output ofthe logic gate. Similarly, when a noise signal has an amplitude lessthan NM_(H), and the logic gate receives a signal at an inputrepresenting the logic high level an including the noise signal,—thenoise signal will be attenuated as the received signal passes from theinput to an output of the logic gate. As it is equally likely for noisesignals to be coupled into signals representing logic low levels andlogic high levels, it is desirable that the noise margins NM_(H) andNM_(L) be substantially equal.

Power supply conductors used to distribute a “positive” power supplyvoltage and a reference or “ground” power supply voltage to logic gatesof a digital logic circuit have finite electrical resistances andinductances. When the outputs of several of the logic gates changesimultaneously, a relatively large switching current “pulse” flowsthrough the power supply conductors. The switching current pulse causesvoltage drops across the electrical resistances and inductances of thepower supply conductors. As a result, the positive power supply voltagelevel at the logic gates with changing outputs, and at neighboring logicgates, is momentarily reduced, or “droops.” At the same time, the groundpower supply voltage level at the logic gates with changing outputs, andat the neighboring logic gates, is momentarily increased, or “bounces.”Such power supply droop and ground bounce may generate noise signalswithin the digital logic circuit. If not attenuated by virtue of noisemargins NM_(H) and NM_(L), the noise signals may cause the digital logiccircuit to produce incorrect output signals.

Dynamic random access memory (DRAM) devices are commonly used to storedata (e.g., within computer systems). Modern synchronous DRAM (SDRAM)devices receive an externally generated clock signal, and use the clocksignal to synchronize operations with other devices (e.g., an SDRAMcontroller). Input signal reception and output signal generation aresynchronized with voltage level transitions (i.e., edges) of the clocksignal. SDRAM devices include multiple “banks” of memory, andperformances of SDRAM devices may be increased by interleaving memoryaccesses among the multiple memory banks in order to hide requiredsignal line precharge times within the SDRAM devices. In addition, inputsignals received by more conventional DRAM devices determine thefunctions performed by the DRAM devices. In contrast, input signalsreceived by SDRAM devices represent commands. The commands may be usedto program registers within the SDRAM devices which control operationsof the SDRAM devices, thus allowing for programmable operation of SDRAMdevices.

A clock signal used to synchronize operations of components of asynchronous digital logic circuit (e.g., including an SDRAM device)alternates periodically between a high voltage level and a low voltagelevel. The high voltage level may be within a voltage range extendingdownward from a maximum operating voltage representing a logic highlevel (e.g., a logic ‘1’ state). Correspondingly, the low voltage levelmay be within a voltage range extending upward from a minimum operatingvoltage representing a logic “low” level (e.g., a logic ‘0’ state). Theoperations of the components are typically synchronized to transitionsof the clock signal between the high voltage level and the low voltagelevel (i.e., rising or falling edges of the clock signal). Accordingly,the clock signal must be distributed to the components such that allcomponents “see” the edges of the clock signal at substantially the sametime. In the manner described above, finite resistances and inductancesof conductors used to distribute clock signals (e.g., clock signalconductors and a ground voltage grid or plane), and/or noise signalscoupled into the conductors, may reduce the high voltage level of theclock signal andlor increase the low voltage level of the clock signal.

An input buffer of a component receiving a clock signal (e.g., a logiccircuit or a clock signal buffer) typically uses a fixed switching pointor “trip point” voltage to produce a “regenerated clock signal.” Theswitching point voltage is typically set to a value mid way betweenselected “ideal” high and low voltage levels of the clock signal. Thereceived clock signal voltage is compared to the switching point voltage(e.g., via a comparator). If the clock signal voltage is greater thanthe switching point voltage, the input buffer may produce theregenerated clock signal within an output voltage range extendingdownward from the maximum operating voltage and representing the logichigh level (e.g., the logic ‘1’ state). On the other hand, if the clocksignal voltage is less than the switching point voltage, the inputbuffer may produce the regenerated clock signal within an output voltagerange extending upward from the minimum operating voltage andrepresenting the logic low level (e.g., the logic ‘0’ state).

Practical clock signals transition between the low voltage level and thehigh voltage level in finite amounts of time (i.e., have finite “risetimes”), and similarly transition between the high voltage level and thelow voltage level in finite lengths of time (i.e., have finite “falltimes”). Characterizing such practical clock signals may involvedetermining a mid voltage level mid way between the high and low voltagelevels, and determining “mid points” of rising and falling edges of theclock signal where the rising and falling edges pass through the midvoltage level. The period of such a practical clock signal may bedefined as an amount of time between a mid point of a rising edge of theclock signal and a mid point of the next rising edge of the clocksignal. The “duty cycle” of a practical clock signal having finite riseand fall times may be defined as a ratio of an amount of time between amid point of a rising edge of the clock signal to a mid point of thenext falling edge to the period of the clock signal.

Where a high voltage level of a clock signal is decreased by an amountwhen traversing a clock distribution network (i.e., due to conductorelectrical characteristics and/or noise signals), and a low voltagelevel of the clock signal is increased by the same amount, an inputbuffer employing the above described method for regenerating the clocksignal by comparing the clock signal voltage to a fixed switching pointvoltage may expectedly produce the regenerated clock signal having thesame duty cycle as the received clock signal. However, in situationswhere the high and low voltage levels of the clock signal are changed bydifferent amounts, and when the high and low voltage levels are bothincreased or both decreased, the regenerated clock signal produced bythe input buffer differs from the duty cycle of the received clocksignal. As a result of such changes in duty cycle, the components of thedigital logic circuit may not “see” the edges of the clock signal atsubstantially the same time. When determining a minimum period of theclock signal, such variations in edge transition times must be accountedfor such that the period of the clock signal is sufficient to allow thedigital logic circuit to produce correct output signals despite thevariations in edge transition times.

It would thus be advantageous to have an input buffer circuit having avariable switching point dependent upon actual high and low voltagelevels of a received input signal (e.g., a clock signal), and not afixed switching point established based upon ideal high and low voltagelevels of the input signal. In a synchronous digital logic system usinga clock signal to synchronize component operations, such an input bufferwould reduce variations in edge transition times of the clock signalreceived by the components, thereby allowing the period of the clocksignal to be reduced, and the performance of the synchronous digitallogic system to be increased.

SUMMARY OF THE INVENTION

An input buffer is presented for buffering an input signal having avoltage magnitude which alternates between a first voltage level and asecond voltage level, where the first and second voltage levels may varyover time. In one embodiment, the input buffer includes a first andsecond detector circuits, an average generator circuit, and adifferential amplifier. The first detector circuit receives the inputsignal and produces a first signal having a magnitude indicative of thefirst voltage level. The second detector circuit also receives the inputsignal, and produces a second signal having a magnitude indicative ofthe second voltage level.

The average generator circuit receives the first and second signals, anduses the magnitudes of the first and second signals to produce a thirdsignal having a magnitude indicative of a third voltage levelsubstantially mid way between the first voltage level and the secondvoltage level. The third voltage level defines a variable anautomatically adjusted “switching point” of the input buffer.

The differential amplifier receives the input signal, the third signal,a first power supply voltage, and a second power supply voltage. Thedifferential amplifier amplifies a difference between the voltagemagnitude of the input signal and the third voltage level in order toproduce an output signal which alternates between the first and secondpower supply voltages.

An integrated circuit is described including the input buffer coupledbetween one of a set of input/output pads and circuitry, wherein thecircuitry may be synchronous dynamic random access memory (SDRAM)circuitry.

The input signal may alternate between the first voltage level and thesecond voltage level periodically, and may have a period and a dutycycle. In this situation, the output signal alternates between the firstand second power supply voltages periodically, and has a periodsubstantially equal to the period of the input signal and a duty cyclesubstantially equal to the duty cycle of the input signal. For example,the input signal may be an input clock signal having a period and a dutycycle, and the output signal may be an output clock signal. In thissituation, the output clock signal alternates between the first andsecond power supply voltages periodically, and has a periodsubstantially equal to the period of the input clock signal and a dutycycle substantially equal to the duty cycle of the input clock signal.

As described above, the third voltage level defines a switching point ofthe input buffer. A first noise margin of the input buffer may existbetween the switching point and the first voltage level of the inputsignal, and a second noise margin of the input buffer may exist betweenthe switching point and the second voltage level of the input signal. Inthis situation, the switching point of the input buffer varies withchanges in the first and second voltage levels of the input signal suchthat the first and second noise margins are maintained substantiallyequal.

For example, the first power supply voltage may be denoted “VDD”, andthe second power supply voltage may be denoted “VSS”, where VDD>VSS. Inaddition, the first voltage level may be greater than the second voltagelevel. Power supply voltage VDD may be greater than or equal to thefirst voltage level, and power supply voltage VSS may be less than orequal to the second voltage level. In this situation, the first noisemargin of the input buffer may be a logic high noise margin NM_(H), andthe second noise margin of the input buffer may be a logic low noisemargin NM_(L). The output signal alternates between VDD and VSS, and theswitching point of the input buffer varies with changes in the first andsecond voltage levels of the input signal such that noise margins NM_(H)and NM_(L) are maintained substantially equal.

In one embodiment of the input buffer, the first signal is a voltagesignal having a voltage magnitude substantially equal to the firstvoltage level, the second signal is a voltage signal having a voltagemagnitude substantially equal to the second voltage level, and the thirdvoltage signal has a voltage magnitude substantially equal to the thirdvoltage level. In this embodiment, the differential amplifier amplifiesa difference between the voltage magnitudes of the input signal and thethird voltage signal in order to produce the output signal.

For example, as described above, the first power supply voltage may bedenoted “VDD”, and the second power supply voltage may be denoted “VSS”,where VDD>VSS. In addition, the first voltage level may be greater thanthe second voltage level. Power supply voltage VDD may be greater thanor equal to the first voltage level, and power supply voltage VSS may beless than or equal to the second voltage level.

The first detector circuit may include a p-channel metal oxidesemiconductor (pMOS) transistor and a capacitor, wherein a sourceterminal of the pMOS transistor receives the input signal, and thecapacitor is coupled between a drain terminal of the pMOS transistor andpower supply voltage VSS. Electrical current flows through the pMOStransistor and charges the capacitor when the voltage magnitude of theinput signal is the first voltage level and a voltage across thecapacitor is substantially less that the first voltage level.

The second detector circuit may include a p-channel metal oxidesemiconductor (pMOS) transistor and a capacitor, wherein a drainterminal of the pMOS transistor receives the input signal, and thecapacitor is coupled between a source terminal of the pMOS transistorand power supply voltage VDD. Electrical current flows through the pMOStransistor and charges the capacitor when the voltage magnitude of theinput signal is the second voltage level and a voltage between thesource terminal and the second power supply voltage is substantiallygreater than the second voltage level.

The average generator circuit may include a pair of resistors connectedin series between the first and second voltage signals, wherein thethird voltage signal is produced at a connection point between the pairof resistors. The pair of resistors may have substantially equalresistance values such that the third voltage signal is substantiallymid way between the first voltage level and the second voltage level.

The differential amplifier may include multiple metal oxidesemiconductor (MOS) transistors connected together to form adifferential network, and an inverter. The differential network mayreceive the input signal and the third voltage signal, and may amplifythe difference between the voltage magnitudes of the input signal andthe third voltage signal to produce an intermediate signal. The invertermay receive the intermediate signal at an input terminal and the firstand second power supply voltages, and produce the output signal at anoutput terminal, wherein the output signal alternates between the firstand second power supply voltages.

A circuit is described including an input/output pad adapted to receivethe input signal, the above input buffer wherein the first and seconddetector circuits and the differential amplifier receive the inputsignal via the input/output pad, and circuitry coupled to receive theoutput signal of the input buffer and configured to perform a functiondependent upon the output signal. The input/output pad, the inputbuffer, and the circuitry may be formed upon and within a singlemonolithic semiconductor substrate, forming an integrated circuit. Asdescribed above, the input signal may be an input clock signal, and theoutput signal may be an output clock signal. The circuitry may be SDRAMcircuitry which uses the output clock signal to synchronize internaloperations.

A method for signal buffering embodied within the input buffer includesreceiving the above input signal, producing the first signal having amagnitude indicative of the first voltage level of the input signal, andproducing the second signal having a magnitude indicative of the secondvoltage level of the input signal. The first and second signals are usedto produce the third signal having a magnitude indicative of a thirdvoltage level substantially mid way between the first voltage level andthe second voltage level of the input signal. The difference between thevoltage magnitude of the input signal and the third voltage level areamplified to produce an output signal which alternates between the firstpower supply voltage and the second power supply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify similar elements, and in which:

FIG. 1 is a diagram of one embodiment of an integrated circuit includingan input buffer coupled between one of a set of electrically conductiveinput/output (I/O) pads and circuitry, wherein the input buffer, the setof I/O pads, and the circuitry are all formed upon and within a singlemonolithic semiconductor substrate, and wherein the input bufferreceives an input clock signal CLK and produces an output clock signalCLK1;

FIG. 2 is a diagram of one embodiment of the input buffer of FIG. 1;

FIG. 3A is a graph of two alternate versions of the input clock signalCLK of FIGS. 1-2 versus time;

FIG. 3B is a graph of two alternate versions of the output clock signalCLKI of FIGS. 1-2 versus time;

FIG. 4 is a diagram of one embodiment of the input buffer of FIGS. 1-2,and wherein the input buffer receives an input signal VIN (e.g., inputclock signal CLK) and produces an output signal VOUT (e.g., output clocksignal CLK1);

FIG. 5 is a graph of input signal VIN and output signal VOUT for theinput buffer of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 0.0 volts (a power supply voltage VSS)and 2.5 volts (a power supply voltage VDD), and TMIN is the time atwhich voltage signal VMID reaches a final value;

FIG. 6 is a graph of input signal VIN and output signal VOUT for theinput buffer of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 1.4 volts and 2.5 volts (VDD), and TMINis the time at which voltage signal VMID reaches a final value;

FIG. 7 is a graph of input signal VIN and output signal VOUT for theinput buffer of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 0.0 volts (VSS) and 0.9 volts, and TMINis the time at which voltage signal VMID reaches a final value; and

FIG. 8 is a graph of input signal VIN and output signal VOUT for theinput buffer of FIG. 4, as well as outputs of an exemplary differentialamplifier and an exemplary inverter receiving input signal VIN, versustime for time≧TMIN, where input signal VIN alternates periodicallybetween 1.0 volts and 2.5 volts (VDD), and TMIN is the time at whichvoltage signal VMID reaches a final value.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will, of course, be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

FIG. 1 is a diagram of one embodiment of an integrated circuit (IC) 100.IC 100 includes an input buffer 102, circuitry 104, and a set ofelectrically conductive input/output (I/O) pads, all formed upon andwithin a single monolithic semiconductor substrate 106. The set of I/Opads may be, for example, flat metal contact regions arranged about anouter edge of substrate 106. The set of I/O pads includes input I/O pads108 and output I/O pads 110. Substrate 106 is housed within a protectivesemiconductor device package 112 having a set of electrically conductiveterminals arranged about an outer surface. The set of terminals mayinclude, for example, “pins” for insertion into holes in a printedcircuit board (PCB), or “leads” for attachment to flat metal contactregions on an exposed surface of a PCB. The set of terminals includesinput terminals 114 and output terminals 116. Each of the inputterminals 114 is electrically connected to a corresponding one of theinput I/O pads 108 (e.g., by a bonding wire), and each of the outputterminals 116 is electrically connected to a corresponding one of theoutput I/O pads 110. It is noted that IC 100 may also include other I/Opads and terminals which are used to receive input signals at sometimes, and to provide output signals at other times.

Input buffer 102 receives an input clock signal CLK via one of the inputI/O pads 108, and produces an output clock signal CLK1, wherein outputclock signal CLK1 may be considered a “regenerated” version of inputclock signal CLK. As described in detail below, input buffer 102continuously monitors a voltage level VHIGH and a voltage level VLOW ofinput clock signal CLK, and produces a voltage signal VMID having avoltage substantially mid way between voltage level VHIGH and voltagelevel VLOW. As described in detail below, voltage signal VMID defines aswitching point, and logic level transitions, of input clock signal CLK.Voltage signal VMID acts as a “switching point voltage” within inputbuffer 102.

Input buffer 102 has a logic high noise margin NM_(H) existing betweenthe switching point voltage VMID and voltage level VHIGH of the inputsignal, and a logic low noise margin NM_(L) existing between switchingpoint voltage VMID and voltage level VLOW of the input signal. As aresult of the continuous adjustment of VMID in response to changes involtage levels VHIGH and/or VLOW, noise margin NM_(H) and noise marginNM_(L) of input buffer 102 are maintained substantially equal. Thisgives input buffer 102 a very high degree of immunity to any noisesignal coupled into input clock signal CLK.

In addition, the continuous adjustment of VMID also results in outputclock signal CLK1 having a duty cycle which is substantially equal to aduty cycle of input clock signal CLK despite any changes in voltagelevels VHIGH and VLOW. Such changes to voltage levels VHIGH and VLOW mayoccur, for example, as input clock signal CLK traverses a clockdistribution network of a synchronous digital logic system on its way toIC 100.

In the embodiment of FIG. 1, circuitry 104 receives output clock signalCLK1 produced by input buffer 102, and other input signals via the otherinput I/O pads 108. Circuitry 104 produces multiple output signalsdependent upon output clock signal CLK1 and one or more of the otherinput signals, and provides a different one of the output signals toeach of the output I/O pads 110. Circuitry 104 may be, for example,synchronous digital logic circuitry. In this situation, reception of theother input signals by circuitry 104 and/or the providing of the outputsignals may be synchronized with voltage level transitions (i.e., edges)of output clock signal CLK1.

As described above, the continuous adjustment of VMID by input buffer102 in response to changes in voltage level VHIGH and/or voltage levelVLOW also results in output clock signal CLK1 having a duty cycle whichis substantially equal to a duty cycle of input clock signal CLK. Outputclock signal CLK1 having a duty cycle substantially equal to a dutycycle of input clock signal CLK means that time periods between edgetransitions of input clock signal CLK are faithfully reproduced inoutput clock signal CLK1. Where circuitry 104 is one of severalcomponents of a system performing operations synchronized by input clocksignal CLK, faithful reproduction of the edge transition times of inputclock signal CLK by input buffer 102 helps ensure that circuitry 104 ofIC 100 “sees” edges of input clock signal CLK at substantially the sametime as the other components.

Circuitry 104 may be, for example, synchronous dynamic random accessmemory (SDRAM) circuitry. The SDRAM circuitry may include variousregisters, and the contents of the various registers may controloperations performed by the SDRAM circuitry. The SDRAM circuitry maysynchronize the reception of the other input signals with voltage leveltransitions (i.e., edges) of output clock signal CLK1. A portion of theother input signals may represent commands, and the commands may be usedto program the registers of the SDRAM circuitry, thus providingprogrammable operation of the SDRAM circuitry. During memory readoperations, the output signals may convey read data, and the SDRAMcircuitry may synchronize the providing of the output signals with theedges of output clock signal CLK1. The SDRAM circuitry may includemultiple memory arrays forming different memory “banks”. The SDRAMcircuitry may be operated such that memory accesses are interleavedamong the multiple memory banks such that required signal line prechargetimes within the various memory banks do not impact the performance ofthe SDRAM circuitry (i.e., are “hidden”).

During fabrication of input buffer 102 and circuitry 104, electricallyconductive signal lines formed upon substrate 106 may be terminated atcorresponding members of the set of I/O pads. Following fabrication ofinput buffer 102 and circuitry 104, substrate 106 may be secured withinpackage 112, and each of the set of I/O pads may be connected to thecorresponding terminal of package 112 (e.g., via a bonding wire).

In other embodiments of IC 100, semiconductor device package 112 mayhave the set of terminals arranged as an array across an undersidesurface. For example, in a ball grid array (BGA) embodiment of IC 100,semiconductor device package 112 may include a second substrate made of,for example, fiberglass-epoxy printed circuit board material or aceramic material (e.g., aluminum oxide, alumina, Al₂O₃, or aluminumnitride, AlN). The second substrate may include two sets of bondingpads: a first set formed upon an upper surface and a second set arrangedas an array across an underside surface of the device package. Thesubstrate may also include one or more layers of electrically conductivesignal lines (i.e., interconnects) used to connect corresponding membersof the first and second sets of bonding pads. Substrate 106 may bemounted upon the upper surface of the second substrate, and each of theset of I/O pads of substrate 106 may be electrically connected tocorresponding members of the first set of bonding pads of the secondsubstrate (e.g., via a bonding wire). Members of the second set ofbonding pads of the second substrate may function as the set ofterminals, and may be coated with solder. The resulting solder “balls”extending from the second set of bonding pads may allow IC 100 to bemounted upon a surface of a PCB.

Where input buffer 102 is used throughout a synchronous digital logicsystem, variations in edge transition times of various versions of asynchronizing clock signal received by components of the system arereduced, thereby allowing the period of the clock signal to be reduced,and the performance of the synchronous digital logic system to beincreased.

FIG. 2 is a diagram of one embodiment of input buffer 102 of FIG. 1. Inthe embodiment of FIG. 2, input buffer includes a positive peak detector200, a differential amplifier 202, a negative peak detector 204, and anaverage value generator 206. Positive peak detector 200 receives inputclock signal CLK, which alternates periodically between a voltage levelVHIGH and a voltage level VLOW. For reasons described above, bothvoltage level VHIGH and voltage level VLOW may vary with time, and mayvary from “nominal” or “ideal” values by different amounts at any giventime. Positive peak detector 200 produces a voltage signal VHIGH whichis substantially equal to voltage level VHIGH of input clock signal CLK.Negative peak detector 204 also receives input clock signal CLK, andproduces a voltage signal VLOW which is substantially equal to thevoltage level VLOW of input clock signal CLK. Average value generator206 receives voltage signal VHIGH from positive peak detector 200 andthe voltage signal VLOW from negative peak detector 204, and produces avoltage signal VMID, where VMID is substantially the average of voltagesignals VHIGH and VLOW. In mathematical terms, VMID≈(VHIGH+VLOW)/2.

Differential amplifier 202 receives input clock signal CLK and voltagesignal VMID, and uses input clock signal CLK and voltage signal VMID toproduce output clock signal CLK1. In general, differential amplifier 202performs an analog function, producing output clock signal CLK1 byamplifying a difference in voltage between input clock signal CLK andvoltage signal VMID. The operation of differential amplifier 202 may beexpressed mathematically as CLK1=k·|(CLK−VMID)|, where CLK is thevoltage of input clock signal CLK, CLK1 is the voltage of the outputclock signal CLK1, and k is the voltage gain of differential amplifier202.

Input clock signal CLK typically alternates between one of two logiclevels. The voltage of input clock signal CLK is within a first voltagerange extending downward from a maximum voltage VMAX when representingone of the logic levels, and is within a second voltage range extendingupward from a minimum operating voltage VMIN when representing the otherlogic level. In the embodiment of FIG. 2, differential amplifier 202preferably receives electrical power via first and second power supplyvoltages. The first power supply voltage is more positive than thesecond, and is preferably substantially equal to VMAX. The second powersupply voltage is preferably substantially equal to VMIN. In thissituation, positive voltage excursions of output clock signal CLK1produced by differential amplifier 202 are expectedly limited by thefirst more positive power supply voltage, and negative voltageexcursions of output clock signal CLK1 are expectedly limited by thesecond power supply voltage. In this way, output clock signal CLK1produced by differential amplifier 202 may be considered a “regenerated”version of input signal CLK.

The rise and fall times of output clock signal CLK1 depend upon thevoltage gain k of differential amplifier 202: the larger the value of k,the shorter the transition times of output signal CLK1. The value of kmay thus be varied to achieve acceptable transition time values. It isalso noted that voltage gain k can be made either positive or negative,depending on a desired “polarity” of output clock signal CLK1.

The operation of differential amplifier 202 may be likened to that of acomparator. For example, differential amplifier 202 may be said to useVMID as a “switching point” voltage, and to “compare” the voltage ofinput clock signal CLK to the switching point voltage VMID. If thevoltage of input clock signal CLK is greater than the switching pointvoltage VMID, the input buffer may produce output clock signal CLK1having a voltage within the first voltage range extending downward frommaximum voltage VMAX and representing one of the two logic levels. Onthe other hand, if the voltage of input clock signal CLK is less thanthe switching point voltage VMID, differential amplifier 202 may produceoutput clock signal CLK1 having a voltage within the second voltagerange extending upward from minimum voltage VMIN and representing theother of the two logic levels.

As noted above, the continuous adjustment of voltage signal VMID inresponse to changes in voltage level VHIGH and/or voltage level VLOWallows input buffer 102 to achieve and maintain substantially equallogic high noise margin NM_(H) and logic low noise margin NM_(L). Thisgives input buffer 102 a very high degree of noise immunity. Inaddition, the continuous adjustment of VMID also results in output clocksignal CLK1 having a duty cycle which is substantially equal to a dutycycle of input clock signal CLK despite changes in voltage levels VHIGHand VLOW.

FIGS. 3A and 3B will now be used to further describe the operation ofinput buffer 102 of FIGS. 1 and 2. FIG. 3A is a graph of two alternateversions of input clock signal CLK of FIGS. 1-2 versus time. Asindicated in FIG. 3A and described above, input clock signal CLKalternates periodically between voltage level VHIGH and voltage levelVLOW. For reasons described above, both voltage levels VHIGH and VLOW ofinput clock signal CLK shown in FIG. 3A may vary with time, and may varyfrom “nominal” or “ideal” values by different amounts at any given time.

As indicated in FIG. 3A, input clock signal CLK has finite rise and falltimes. Average value generator 206 (FIG. 2) produces voltage signalVMID, where VMID is substantially mid way between voltage level VHIGHand voltage level VLOW. In a first version 300 of input clock signal CLKin FIG. 3A, the voltage of input clock signal CLK is equal to voltagelevel VHIGH during a first portion of each period of the input clocksignal CLK, and the voltage of input clock signal CLK is equal tovoltage level VLOW during a remainder of each period. Version 300 ofinput clock signal CLK may be said to be “active high,” and tocorrespond to positive logic.

In a second version 302 of input clock signal CLK in FIG. 3A, thevoltage of input clock signal CLK is equal to voltage level VLOW duringthe first portion of each period of the input clock signal CLK, and thevoltage of input clock signal CLK is equal to voltage level VHIGH duringthe remainder of each period. Version 300 of input clock signal CLK maybe said to be “active low,” and to correspond to negative logic.

FIG. 3B is a graph of two alternate versions of output clock signal CLK1of FIGS. 1-2 versus time. As described above, output clock signal CLK1produced by input buffer 102 may be considered a “regenerated” versionof input clock signal CLK. As evident in FIG. 3B, output clock signalCLK1 alternates periodically between a high voltage level VH and lowvoltage level VL, where VH>VL. A voltage level VM is also shown in FIG.3B, where VM is substantially mid way between VH and VL. As shown inFIG. 3B, output clock signal CLK1 is delayed from input clock signal CLKby a propagation time “t_(p).”

As described above, both voltage levels VHIGH and VLOW of input clocksignal CLK shown in FIG. 3A may vary with time, and may vary from“nominal” or “ideal” values by different amounts at any given time. In“regenerating” input clock signal CLK, input buffer 102 referablyproduces output clock signal CLK1 such that voltage levels VH and VL ofoutput lock signal CLK1 are closer to the “nominal” or “ideal” voltagelevels.

In a first version 304 of output clock signal CLK1 in FIG. 3B, thevoltage of output clock signal CLK1 is equal to voltage level VH duringa first portion of each period of the output clock signal CLK1, and thevoltage of output clock signal CLK1 is equal to voltage level VL duringa remainder of each period. Version 304 of output clock signal CLK1 maybe said to be “active high,” and to correspond to positive logic.

In a second version 306 of output clock signal CLK1 in FIG. 3B, thevoltage of output clock signal CLK1 is equal to voltage level VL duringthe first portion of each period of the output clock signal CLK1, andthe voltage of output clock signal CLK1 is equal to voltage level VHduring the remainder of each period. Version 306 of output clock signalCLK1 may be said to be “active low,” and to correspond to negativelogic.

It is noted that input buffer 102 may produce either first version 304or second version 306 of output clock signal CLK1 from first version 300of input clock signal CLK (FIG. 3A). For example, in the embodiment ofinput buffer 102 of FIG. 2, voltage gain k of differential amplifier 202may be made positive or negative to produce either first version 304 orsecond version 306 of output clock signal CLK1. In a similar manner,input buffer 102 can be made to produce either first version 304 orsecond version 306 of output clock signal CLK1 from second version 302of input clock signal CLK (FIG. 3A).

Using high gain amplification of the voltage difference between inputclock signal CLK and “switching point” voltage signal VMID, input buffer102 may be able to reduce the transition times of output clock signalCLK1 with respect to those of input clock signal CLK. Such transitiontime reduction is evident in the faster voltage level transitions ofoutput clock signal CLK1 in FIG. 3B as compared to the slowertransitions of input clock signal CLK in FIG. 3A.

Voltage signal VMID (FIG. 3A) and voltage level VM (FIG. 3B) are eachsubstantially mid way between the high and low voltage levels of therespective clock signals, and may be used to determine “mid points” ofvoltage level transitions (i.e., edges) of the respective clock signals.As defined herein, the “period” of a signal alternating between twodifferent voltage levels is defined as a time interval between a midpoint of a rising (or falling) edge of the signal and a mid point of thenext rising (or falling) edge of the clock signal. The “duty cycle” ofthe signal is defined herein as a ratio of a time interval between a midpoint of a rising (or falling) edge of the signal and a mid point of thenext falling (or rising) edge to the period of the signal.

According to the above definitions, input clock signal CLK in FIG. 3Ahas a period equal to t₂ and a duty cycle equal to (t₁/t₂). Output clocksignal CLK1 in FIG. 3B has a period t₄, where t₄≡t₂, and a duty cycle of(t₃/t₄), where t₃≡t₁. Thus the duty cycle of output clock signal CLK1 issubstantially equal to the duty cycle of input clock signal CLK. Asdescribed above, this result is made possible by the continuousadjustment of voltage signal VMID by input buffer 102 in response tochanges in voltage levels VHIGH and VLOW of input clock signal CLK.

FIG. 4 is a diagram of one embodiment of input buffer 102 of FIGS. 1-2.In the embodiment of FIG. 4, positive peak detector 200 includes anenhancement mode, p-channel metal oxide semiconductor (pMOS) transistor402 having a source terminal and a substrate (bulk) electricallyconnected to one another and to a node receiving input signal VIN (e.g.,input clock signal CLK in FIGS. 1 and 2). Input signal VIN alternatesbetween voltage level VHIGH and voltage level VLOW as shown in FIG. 3A.However, input signal VIN may not be periodic, and may not be asynchronizing clock signal. For example, input signal VIN may be aserial data stream which conveys serial data by alternating betweenvoltage levels VHIGH and VLOW. For this reason, the input signal toinput buffer 102 will be referred to hereinbelow as input signal VIN.

A gate terminal and a drain terminal of pMOS transistor 402 areconnected to one another and to a first terminal of a capacitor 404. Asecond terminal of capacitor 404 is connected to a power supply voltageVSS which serves as a reference voltage (i.e., a ground voltage). Aunity gain buffer 406 has an input terminal connected to the firstterminal of capacitor 404, and produces a voltage signal VHIGH at anoutput terminal.

In the embodiment of FIG. 4, unity gain buffer 406 is an operationalamplifier. A positive input terminal of the operational amplifier is theinput terminal of unity gain buffer 406, and an output terminal of theoperational amplifier is the output terminal of unity gain buffer 406. Anegative input terminal of the operational amplifier is connected to theoutput terminal, providing a feedback path for the operationalamplifier. Other embodiments of unity gain buffer 406 are possible andcontemplated.

Capacitor 404 has a voltage VC404 between the first and secondterminals, with VC404 positive from the first terminal to the secondterminal. When VIN>(VC404−VTP), where VTP is a threshold voltage of pMOStransistor 402 and (VTP<0), a channel having a relatively low electricalresistance is formed between the source and drain terminals of pMOStransistor 402. This typically occurs when VIN is the voltage levelVHIGH as shown in FIG. 3. An electrical current flows through pMOStransistor 402 and into the first terminal of capacitor 404, chargingcapacitor 404 and increasing voltage VC404. In general, the voltage atthe input terminal of unity gain buffer 406, voltage VC404 of capacitor404, increases toward a maximum value of (VHIGH+VTP) when VIN is thevoltage level VHIGH as shown in FIG. 3. Thus the voltage at the inputterminal of unity gain buffer 406 tends to “follow” voltage level VHIGHof input signal VIN, and a final value of voltage signal VHIGH issubstantially equal to voltage level VHIGH.

When VIN<(VC404−VTP), the channel does not exist, and the electricalresistance between the source and drain terminals of pMOS transistor 402is relatively high. This typically occurs when VIN is the voltage levelVLOW as shown in FIG. 3. In general, voltage VC404 of capacitor 404, thevoltage at the input terminal of unity gain buffer 406, does not changesubstantially when VIN is the voltage level VLOW as shown in FIG. 3.

In the embodiment of FIG. 4, negative peak detector 204 includes anenhancement mode pMOS transistor 408 having a drain terminal and a gateterminal connected to one another and to the node receiving input signalVIN. A source terminal and a substrate (bulk) of pMOS transistor 408 areelectrically connected to one another and to a first terminal of acapacitor 410. A second terminal of capacitor 410 is connected to apower supply voltage VDD, which is greater than power supply voltageVSS. A unity gain buffer 412 has an input terminal connected to thefirst terminal of capacitor 410, and produces a voltage signal VLOW atan output terminal.

In the embodiment of FIG. 4, unity gain buffer 412 is an operationalamplifier. A positive input terminal of the operational amplifier is theinput terminal of unity gain buffer 412, and an output terminal of theoperational amplifier is the output terminal of unity gain buffer 412. Anegative input terminal of the operational amplifier is connected to theoutput terminal, providing a feedback path for the operationalamplifier. Other embodiments of unity gain buffer 412 are possible andcontemplated.

Capacitor 410 has a voltage VC410 between the first and secondterminals, with VC410 positive from the first terminal to the secondterminal. When VIN<(VTP+VC410+VDD), where VTP is the threshold voltageof pMOS transistor 408 and (VTP<0), a channel having a relatively lowelectrical resistance is formed between the source and drain terminalsof pMOS transistor 408. This typically occurs when VIN is the voltagelevel VLOW as shown in FIG. 3. An electrical current flows through pMOStransistor 408 and out of the first terminal of capacitor 410, chargingcapacitor 410 and increasing voltage VC410. In general, voltage(VC410+VDD) at the input terminal of unity gain buffer 412 decreasestoward a minimum value of (VLOW−VTP) when VIN is the voltage level VLOWas shown in FIG. 3. Thus the voltage at the input terminal of unity gainbuffer 412 tends to “follow” voltage level VLOW of input signal VIN, anda final value of voltage signal VLOW is substantially equal to voltagelevel VLOW.

When VIN>(VTP+VC410+VDD), the channel does not exist, and the electricalresistance between the source and drain terminals of pMOS transistor 408is relatively high. This typically occurs when VIN is the voltage levelVHIGH as shown in FIG. 3. In general, voltage VC410 of capacitor 410,and the voltage at the input terminal of unity gain buffer 412, do notchange substantially when VIN is the voltage level VHIGH as shown inFIG. 3.

In the embodiment of FIG. 4, average value generator 206 includes afirst resistor 414, a second resistor 416, and a capacitor 418.Resistors 414 and 416 are connected in series between the VHIGH signalproduced by positive peak detector 200 and the VLOW signal produced bynegative peak detector 204. A voltage signal VMID is produced at a nodewhere resistors 414 and 416 are connected to one another. Resistors 414and 416 have substantially the same resistance values and form a voltagedivider network such that VMID is substantially mid way between, orsubstantially the average of, VHIGH and VMID. Mathematically,VMID≈(VHIGH+VLOW)/2. A first terminal of capacitor 418 is connected tothe node between resistors 414 and 416, and a second terminal ofcapacitor 418 is connected to power supply voltage VSS. Capacitor 418 isused to “smooth” transitions in VMID when VHIGH and/or VLOW change.

Differential amplifier 202 includes a differential network 420.Differential network 420 includes two enhancement mode, n-channel metaloxide semiconductor (nMOS) driver transistors 422 and 424 having sourceterminals connected together at a node to form a “source-coupled pair.”Each nMOS driver transistor 422 and 424 has a corresponding pMOStransistor acting as an active load. An enhancement mode pMOS transistor426 connected in series with nMOS driver transistor 422 forms an activeload for nMOS driver transistor 422, and an enhancement mode pMOStransistor 428 connected in series with nMOS driver transistor 424 formsan active load for nMOS driver transistor 428. Aspect ratios of nMOSdriver transistors 422 and 424 are preferably substantially the same(i.e., nMOS transistors 422 and 424 are preferably “matched”transistors).

The pMOS load transistors 426 and 428 are connected together to form acurrent mirror. Source terminals of pMOS transistors 426 and 428 areconnected to power supply voltage VDD. A gate terminal of pMOStransistor 426 is connected to a drain terminal of pMOS transistor 426and to a gate terminal of pMOS transistor 428 such that a currentflowing from source-to-drain through pMOS transistor 428 is directlyproportional to a current flowing from source-to-drain through pMOStransistor 426. Aspect ratios of pMOS transistors 426 and 428 arepreferably substantially the same (i.e., pMOS transistors 426 and 428are preferably “matched”) such that currents flowing through pMOStransistors 426 and 428 are substantially equal.

Differential network 420 also includes a constant current sink 430 whichdraws a substantially constant current out of the node where the sourceterminals of nMOS drive transistors 422 and 424 are connected together.Constant current sink 430 includes enhancement mode nMOS transistors 432and 434 connected together to form a current mirror. Source terminals ofnMOS transistors 432 and 434 are connected to power supply voltage VSS.A gate terminal of nMOS transistor 432 is connected to a drain terminalof nMOS transistor 432 and to a gate terminal of nMOS transistor 434such that a current flowing from drain-to-source through nMOS transistor434 is directly proportional to a current flowing from drain-to-sourcethrough nMOS transistor 432. A resistor 436 connected between powersupply voltage VDD and the drain terminal of nMOS transistor 432determines the drain-to-source current through nMOS transistor 432.Aspect ratios of nMOS transistors 432 and 434 may be made substantiallythe same (i.e., nMOS transistors 432 and 434 may be “matched”) such thatcurrents flowing through pMOS transistors 426 and 428 are substantiallyequal.

A gate terminal of nMOS driver transistor 422 is connected to the nodereceiving input signal VIN, and a gate terminal of nMOS drivertransistor 424 is connected to the node of average value generator 206where resistors 414 and 416 are connected to one another, and receivesvoltage signal VMID. At a node N where the drain terminal of nMOS driver424 is connected to the drain terminal of pMOS load transistor 428, avoltage signal VN is produced which is the amplified difference betweeninput signal VIN and voltage signal VMID. Mathematically,VN=k·(VIN−VMID) where k is a voltage gain of differential network 420.It is noted that in the embodiment of FIG. 4, voltage gain k is positiveand much greater than 1.

In the embodiment of FIG. 4, differential amplifier 202 also includes aninverter 438 having an input terminal connected to node N and an outputterminal producing an output signal VOUT (e.g., output clock signalCLK1). Inverter 438 may be, for example, a complementary metal oxidesemiconductor (CMOS) inverter including an enhancement mode pMOStransistor and an enhancement mode nMOS transistor connected in seriesbetween power supply voltages VDD and VSS. In such a CMOS inverter, gateterminals of the nMOS and pMOS transistors receive voltage signal VN.Inverter 438 preferably has a switching point substantially equal toVDD/2.

The operation of differential amplifier 202 may be likened to that of acomparator. For example, differential amplifier 202 may be said to usevoltage signal VMID as a “switching point” voltage, and to “compare”input signal VIN to the switching point voltage VMID. In the embodimentof FIG. 4, if VIN is greater than the switching point voltage VMID,differential amplifier 202 produces output signal VOUT within a “low”voltage range extending upward from reference power supply voltage VSSand representing one of two logic levels (e.g., a logic 0). On the otherhand, if VIN is less than the switching point voltage VMID, differentialamplifier 202 produces output signal VOUT within a “high” voltage rangeextending downward from power supply voltage VDD and representing theother of the two logic levels (e.g., a logic 1).

For simplicity, the substrate (bulk) connections of the MOS transistorsmaking up differential amplifier 202 have not been shown in FIG. 4, andthe MOS transistor symbols used in differential amplifier 202 reflectthis fact. The substrates (bulks) of all nMOS transistors ofdifferential amplifier 202 are connected to power supply voltage VSS,and the substrates (bulks) of all pMOS transistors of differentialamplifier 202 are connected to power supply voltage VDD.

It is noted that as voltage gain k of differential network 420 ispositive and inverter 438 inverts signal VN, output signal VOUT isinverted with respect to input signal VIN. It is also noted that inother embodiments of differential amplifier 202, differential network420 can be configured such that voltage gain k of differential network420 is made negative, thus changing the polarity of output signal VOUTsuch that output signal VOUT is in phase with input signal VIN.

As noted above, the continuous adjustment of voltage signal VMID inresponse to changes in VHIGH and/or VLOW allows input buffer 102 toachieve and maintain substantially equal logic high noise margin NM_(H)and logic low noise margin NM_(L). This gives input buffer 102 a veryhigh degree of noise immunity. In addition, where input signal VIN is aperiodic clock signal (e.g., input clock signal CLK) and output signalVOUT is a “regenerated” version of that periodic clock signal (e.g.,output clock signal CLK1), the continuous adjustment of voltage signalVMID results in output signal VOUT (CLK1) having a duty cycle which issubstantially equal to a duty cycle of input signal VIN (CLK) despitechanges in voltage levels VHIGH and VLOW of input signal VIN.

FIG. 5 is a graph of input signal VIN and output signal VOUT for inputbuffer 102 of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 0.0 volts (VSS) and 2.5 volts (VDD), andTMIN is the time at which voltage signal VMID reaches a final value.Input signal VIN may be, for example, a synchronizing clock signal. Asshown in FIG. 5 and defined above, the period of input signal VIN is 20nanoseconds, corresponding to a frequency of 50 MHz. Input signal VINhas rise and fall times of 2 nanoseconds, resulting in a trapezoidalwaveform. The duty cycle of input signal VIN as defined above is about50 percent.

As shown in FIG. 5, output signal VOUT also alternates periodicallybetween 0.0 volts (VSS) and 2.5 volts (VDD), and is inverted withrespect to input signal VIN. Like input signal VIN, output signal VOUTalso has a period of about 20 nanoseconds as defined above,corresponding to a frequency of 50 MHz. Output signal VOUT has rise andfall times much less than the 2 nanosecond rise and fall times of inputsignal VIN. As a result, output signal VOUT more closely resembles asquare wave than input signal VIN. The duty cycle of output signal VOUTas defined above is also about 50 percent.

It is noted that capacitor voltages VC404 and VC410 across respectivecapacitors 404 and 410 in FIG. 4 are zero at time=0, and input signalVIN is applied to input buffer 102 at time=0. Voltage signal VMIDproduced by average value generator 206 (FIG. 4) is substantially equalto VDD/2, or 1.25 volts, at time=0. Voltage signal VHIGH produced bypositive peak detector 200 (FIG. 4) increases from 0.0 volts toward afinal value of (2.5 volts+VTP) where (VTP<0), and voltage signal VLOWproduced by negative peak detector 204 decreases from 2.5 volts toward afinal value of (0.0 volts−VTP).

The rate at which voltage signal VHIGH increases may be different thanthe rate at which voltage signal VLOW decreases. In this situation,voltage signal VMID may be greater than or less than the final value ofVDD/2 (1.25 volts) between time=0 and time=TMIN. By time=TMIN, VMID hasreached (i.e., returned to) the final value, and remains substantiallythe final value for time≧TMIN. In this situation, TMIN>0 (e.g., a fewhundred nanoseconds). Alternately, the rate at which voltage signalVHIGH increases may be substantially equal to the rate at which voltagesignal VLOW decreases such that VMID remains substantially equal to thefinal value of VDD/2 (1.25 volts) for all time≧0. In this situation,TMIN=0.

FIG. 6 is a graph of input signal VIN and output signal VOUT for inputbuffer 102 of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 1.4 volts and 2.5 volts (VDD), and TMINis the time at which voltage signal VMID reaches a final value. Powersupply voltage VSS is 0.0 volts. As shown in FIG. 5 and defined above,the period of input signal VIN is 20 nanoseconds, corresponding to afrequency of 50 MHz. Input signal VIN has rise and fall times of 2nanoseconds, resulting in a trapezoidal waveform. The duty cycle ofinput signal VIN as defined above is about 50 percent.

As shown in FIG. 6, output signal VOUT alternates periodically between0.0 volts (VSS) and 2.5 volts (VDD), despite that fact that input signalVIN does not. Output signal VOUT is inverted with respect to inputsignal VIN as noted above. Like input signal VIN, output signal VOUT hasa period of about 20 nanoseconds as defined above, corresponding to afrequency of 50 MHz. Output signal VOUT has rise and fall times muchless than the 2 nanosecond rise and fall times of input signal VIN. As aresult, output signal VOUT more closely resembles a square wave thaninput signal VIN. The duty cycle of output signal VOUT as defined aboveis about 49 percent, substantially the 50 percent duty cycle of inputsignal VIN.

Again, capacitor voltages VC404 and VC410 across respective capacitors404 and 410 in FIG. 4 are zero at time=0, and input signal VIN isapplied to input buffer 102 at time=0. Voltage signal VMID produced byaverage value generator 206 (FIG. 4) is substantially equal to VDD/2, or1.25 volts, at time=0. Voltage signal VHIGH produced by positive peakdetector 200 (FIG. 4) increases from 0.0 volts toward a final value of(2.5 volts+VTP) where (VTP<0), and voltage signal VLOW produced bynegative peak detector 204 decreases from 2.5 volts toward a final valueof (1.4 volts−VTP). The rate at which voltage signal VHIGH increases isgreater than the rate at which voltage signal VLOW decreases, andvoltage signal VMID increases from 1.25 volts toward a final value(e.g., 1.95 volts). Voltage signal VMID reaches the final value attime=TMIN (e.g., a few hundred nanoseconds), and VMID is substantiallyequal to the final value for time≧TMIN.

FIG. 7 is a graph of input signal VIN and output signal VOUT for inputbuffer 102 of FIG. 4 versus time for time≧TMIN, where input signal VINalternates periodically between 0.0 volts (VSS) and 0.9 volts, and TMINis the time at which voltage signal VMID reaches a final value. Powersupply voltage VDD is 2.5 volts. As shown in FIG. 5 and defined above,the period of input signal VIN is 20 nanoseconds, corresponding to afrequency of 50 MHz. Input signal VIN has rise and fall times of 2nanoseconds, resulting in a trapezoidal waveform. The duty cycle ofinput signal VIN as defined above is about 50 percent.

As shown in FIG. 7, output signal VOUT alternates periodically between0.0 volts (VSS) and 2.5 volts (VDD), despite that fact that input signalVIN does not. Output signal VOUT is inverted with respect to inputsignal VIN as noted above. Like input signal VIN, output signal VOUT hasa period of about 20 nanoseconds as defined above, corresponding to afrequency of 50 MHz. Output signal VOUT has rise and fall times muchless than the 2 nanosecond rise and fall times of input signal VIN. As aresult, output signal VOUT more closely resembles a square wave thaninput signal VIN. The duty cycle of output signal VOUT as defined aboveis about 50 percent, the same as the duty cycle of input signal VIN.

Again, capacitor voltages VC404 and VC410 across respective capacitors404 and 410 in FIG. 4 are zero at time=0, and input signal VIN isapplied to input buffer 102 at time=0. Voltage signal VMID produced byaverage value generator 206 (FIG. 4) is substantially equal to VDD/2, or1.25 volts, at time=0. Voltage signal VHIGH produced by positive peakdetector 200 (FIG. 4) increases from 0.0 volts toward a final value of(0.9 volts+VTP) where (VTP<0), and voltage signal VLOW produced bynegative peak detector 204 decreases from 2.5 volts toward a final valueof (0.0 volts−VTP). The rate at which voltage signal VHIGH increases isless than the rate at which voltage signal VLOW decreases, and voltagesignal VMID decreases from 1.25 volts toward a final value (e.g., 0.48volts). Voltage signal VMID reaches the final value at time=TMIN (e.g.,a few hundred nanoseconds), and VMID is substantially equal to the finalvalue for time≧TMIN.

FIG. 8 is a graph of input signal VIN and output signal VOUT for inputbuffer 102 of FIG. 4, as well as outputs of an exemplary differentialamplifier and an exemplary inverter receiving input signal VIN, versustime for time≧TMIN, where input signal VIN alternates periodicallybetween 1.0 volts and 2.5 volts (VDD), and TMIN is the time at whichvoltage signal VMID reaches a final value. Power supply voltage VSS is0.0 volts. As shown in FIG. 8 and defined above, the period of inputsignal VIN is 20 nanoseconds, corresponding to a frequency of 50 MHz.Input signal VIN has rise and fall times of 4 nanoseconds, resulting ina markedly trapezoidal waveform. The duty cycle of input signal VIN asdefined above is about 42 percent.

The exemplary differential amplifier is similar to differential network420 shown in FIG. 4 and described above. A gate terminal of an nMOSdriver transistor corresponding to nMOS driver transistor 422 of FIG. 4receives input signal VIN, and a gate terminal of an nMOS drivertransistor corresponding to nMOS driver transistor 424 is connected to a1.25 volt reference voltage. The switching point of the exemplarydifferential amplifier is thus 1.25 volts. The output signal of theexemplary differential amplifier is sampled at the node corresponding tonode N of FIG. 4 where voltage signal VN is produced. Mathematically,the output signal voltage of the exemplary differential amplifier isk·(VIN−1.25 volts) where k is the voltage gain of the exemplarydifferential amplifier, and k is positive and much greater than 1.

The exemplary inverter is a CMOS inverter including an enhancement modepMOS transistor and an enhancement mode nMOS transistor connected inseries between power supply voltages VDD and VSS. Gate terminals of thenMOS and pMOS transistors receive input signal VIN. The exemplaryinverter has a switching point of 1.20 volts.

As shown in FIG. 8, output signal VOUT produced by input buffer 102alternates periodically between 0.0 volts (VSS) and 2.5 volts (VDD),despite the fact that input signal VIN does not. Output signal VOUT isinverted with respect to input signal VIN as noted above. Like inputsignal VIN, output signal VOUT has a period of about 20 nanoseconds asdefined above, corresponding to a frequency of 50 MHz. Output signalVOUT has rise and fall times much less than the 4 nanosecond rise andfall times of input signal VIN. As a result, output signal VOUT moreclosely resembles a square wave than input signal VIN. The duty cycle ofoutput signal VOUT as defined above is about 42 percent, the same as theduty cycle of input signal VIN.

Again, capacitor voltages VC404 and VC410 across respective capacitors404 and 410 in FIG. 4 are zero at time=0, and input signal VIN isapplied to input buffer 102 at time=0. Voltage signal VMID produced byaverage value generator 206 (FIG. 4) is substantially equal to VDD/2, or1.25 volts, at time=0. Voltage signal VHIGH produced by positive peakdetector 200 (FIG. 4) increases from 0.0 volts toward a final value of(2.5 volts+VTP) where (VTP<0), and voltage signal VLOW produced bynegative peak detector 204 decreases from 2.5 volts toward a final valueof (1.0 volts−VTP). The rate at which voltage signal VHIGH increases isgreater than the rate at which voltage signal VLOW decreases, andvoltage signal VMID increases from 1.25 volts toward a final value(e.g., 1.74 volts). Voltage signal VMID reaches the final value attime=TMIN (e.g., a few hundred nanoseconds), and VMID is substantiallyequal to the final value for time≧TMIN.

The output of the exemplary differential amplifier alternatesperiodically between about 0.4 volts and 2.5 volts as indicted in FIG.8. The output of the exemplary differential amplifier is in phase withinput signal VIN. Like input signal VIN, the output of the exemplarydifferential amplifier has a period of about 20 nanoseconds as definedabove, corresponding to a frequency of 50 MHz. The output of theexemplary differential amplifier has rise and fall times much less thanthe 4 nanosecond rise and fall times of input signal VIN, and moreclosely resembles a square wave than input signal VIN. The duty cycle ofthe output of the exemplary differential amplifier output signal VOUT asdefined above is about 54 percent, which differs markedly from the 42percent duty cycle of input signal VIN.

The output of the exemplary inverter alternates periodically between 0.0volts and about 1.8 volts as indicated in FIG. 8. Like output signalVOUT of input buffer 102, the output of the exemplary inverter isinverted with respect to input signal VIN. Like input signal VIN, theoutput of the exemplary inverter has a period of about 20 nanoseconds asdefined above, corresponding to a frequency of 50 MHz. The output of theexemplary inverter has rise and fall times much less than the 4nanosecond rise and fall times of input signal VIN, and more closelyresembles a square wave than input signal VIN. The duty cycle of theoutput of the exemplary inverter as defined above is about 58 percent,which differs markedly from the 42 percent duty cycle of input signalVIN.

FIG. 8 serves to illustrate that where input signal VIN is a periodicclock signal (e.g., input clock signal CLK) and output signal VOUT is a“regenerated” version of that periodic clock signal (e.g., output clocksignal CLK1), the continuous adjustment of voltage signal VMID resultsin output signal VOUT (CLK1) having a duty cycle which is substantiallyequal to a duty cycle of input signal VIN (CLK) despite changes involtage levels VHIGH and VLOW of input signal VIN. It is also noted thatthe continuous adjustment of VMID in response to changes in voltagelevels VHIGH and/or VLOW of input signal VIN allows input buffer 102 toachieve and maintain substantially equal logic high noise margin NM_(H)and logic low noise margin NM_(L). This gives input buffer 102 a veryhigh degree of noise immunity.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

What is claimed is:
 1. An input buffer, comprising: a first detectorcircuit adapted to receive an input signal, wherein a voltage magnitudeof the input signal alternates between a first voltage level and asecond voltage level, and wherein the first detector circuit isconfigured to produce a first signal having a magnitude indicative ofthe first voltage level; a second detector circuit adapted to receivethe input signal and configured to produce a second signal having amagnitude indicative of the second voltage level; an average generatorcircuit adapted to receive the first and second signals and configuredto use the magnitudes of the first and second signals to produce a thirdsignal having a magnitude indicative of a third voltage levelsubstantially mid way between the first voltage level and the secondvoltage level; and a differential amplifier adapted to receive the inputsignal, the third signal, a first power supply voltage, and a secondpower supply voltage, wherein the differential amplifier is configuredto amplify a difference between the voltage magnitude of the inputsignal and the third voltage level in order to produce an output signalwhich alternates between the first and second power supply voltages. 2.The input buffer as recited in claim 1, wherein the input signalalternates between the first voltage level and the second voltage levelperiodically and has a period and a duty cycle, and wherein the outputsignal alternates between the first and second power supply voltagesperiodically and has a period substantially equal to the period of theinput signal and a duty cycle substantially equal to the duty cycle ofthe input signal.
 3. The input buffer as recited in claim 1, wherein thethird voltage level defines a switching point of the input buffer, andwherein a first noise margin of the input buffer exists between theswitching point and the first voltage level of the input signal, andwherein a second noise margin of the input buffer exists between theswitching point and the second voltage level of the input signal, andwherein the switching point of the input buffer varies with changes inthe first and second voltage levels of the input signal such that thefirst and second noise margins are maintained substantially equal. 4.The input buffer as recited in claim 1, wherein the first voltage levelis greater than the second voltage level, and wherein the first powersupply voltage is greater than or equal to the first voltage level, andwherein second power supply voltage is less than or equal to the secondvoltage level.
 5. The input buffer as recited in claim 1, wherein thefirst signal is a voltage signal having a voltage magnitudesubstantially equal to the first voltage level.
 6. The input buffer asrecited in claim 1, wherein the second signal is a voltage signal havinga voltage magnitude substantially equal to the second voltage level. 7.The input buffer as recited in claim 1, wherein the third voltage signalhas a voltage magnitude substantially equal to the third voltage level.8. The input buffer as recited in claim 7, wherein the differentialamplifier is configured to amplify a difference between the voltagemagnitudes of the input signal and the third voltage signal in order toproduce the output signal.
 9. An input buffer, comprising: a firstdetector circuit adapted to receive an input signal, wherein a voltagemagnitude of the input signal alternates between a first voltage leveland a second voltage level, and wherein the first detector circuit isconfigured to produce a first voltage signal having a voltage magnitudesubstantially equal to the first voltage level; a second detectorcircuit adapted to receive the input signal and configured to produce asecond voltage signal having a voltage magnitude substantially equal tothe second voltage level; an average generator circuit adapted toreceive the first and second voltage signals and configured to use thevoltage magnitudes of the first and second voltage signals to produce athird voltage signal, wherein the third voltage signal has a voltagemagnitude substantially mid way between the first voltage level and thesecond voltage level; and a differential amplifier adapted to receivethe input signal, the third voltage signal, a first power supplyvoltage, and a second power supply voltage, wherein the differentialamplifier is configured to amplify a difference between the voltagemagnitudes of the input signal and the third voltage signal in order toproduce an output signal which alternates between the first and secondpower supply voltages.
 10. The input buffer as recited in claim 9,wherein the input signal alternates between the first voltage level andthe second voltage level periodically and has a period and a duty cycle,and wherein the output signal alternates between the first and secondpower supply voltages periodically and has a period substantially equalto the period of the input signal and a duty cycle substantially equalto the duty cycle of the input signal.
 11. The input buffer as recitedin claim 9, wherein the voltage magnitude of the third voltage signaldefines a switching point of the input buffer, and wherein a first noisemargin of the input buffer exists between the switching point and thefirst voltage level of the input signal, and wherein a second noisemargin of the input buffer exists between the switching point and thesecond voltage level of the input signal, and wherein the switchingpoint of the input buffer varies with changes in the first and secondvoltage levels of the input signal such that the first and second noisemargins are maintained substantially equal.
 12. The input buffer asrecited in claim 9, wherein the first voltage level is greater than thesecond voltage level, and wherein the first power supply voltage isgreater than or equal to the first voltage level, and wherein secondpower supply voltage is less than or equal to the second voltage level.13. The input buffer as recited in claim 12, wherein the first detectorcircuit comprises a p-channel metal oxide semiconductor (pMOS)transistor and a capacitor, and wherein a source terminal of the pMOStransistor is coupled to receive the input signal, and wherein thecapacitor is coupled between a drain terminal of the pMOS transistor andthe second power supply voltage, and wherein electrical current flowsthrough the pMOS transistor and charges the capacitor when the voltagemagnitude of the input signal is the first voltage level and a voltageacross the capacitor is substantially less that the first voltage level.14. The input buffer as recited in claim 12, wherein the second detectorcircuit comprises a p-channel metal oxide semiconductor (pMOS)transistor and a capacitor, and wherein a drain terminal of the pMOStransistor is coupled to receive the input signal, and wherein thecapacitor is coupled between a source terminal of the pMOS transistorand the first power supply voltage, and wherein electrical current flowsthrough the pMOS transistor and charges the capacitor when the voltagemagnitude of the input signal is the second voltage level and a voltagebetween the source terminal and the second power supply voltage issubstantially greater than the second voltage level.
 15. The inputbuffer as recited in claim 12, wherein the average generator circuitcomprises a pair of resistors connected in series between the first andsecond voltage signals, and wherein the third voltage signal is producedat a connection point between the pair of resistors, and wherein thepair of resistors have substantially equal resistance values such thatthe third voltage signal is substantially mid way between the firstvoltage level and the second voltage level.
 16. The input buffer asrecited in claim 12, wherein the differential amplifier comprises aplurality of metal oxide semiconductor (MOS) transistors, connectedtogether to form a differential network, and an inverter, and whereinthe differential network is coupled to receive the input signal and thethird voltage signal and configured to amplify the difference betweenthe voltage magnitudes of the input signal and the third voltage signalto produce an intermediate signal, and wherein the inverter is coupledto receive the intermediate signal at an input terminal and the firstand second power supply voltages, and wherein the inverter is configuredto produce the output signal at an output terminal, and wherein theoutput signal alternates between the first and second power supplyvoltages.
 17. A circuit, comprising: an input/output pad adapted toreceive an input signal, wherein a voltage magnitude of the input signalalternates between a first voltage level and a second voltage level; aninput buffer, comprising: a first detector circuit coupled to theinput/output pad and configured to produce a first signal having amagnitude indicative of the first voltage level of the input signal; asecond detector circuit coupled to the input/output pad and configuredto produce a second signal having a magnitude indicative of the secondvoltage level of the input signal; an average generator circuit coupledto receive the first and second signals and configured to use themagnitudes of the first and second signals to produce a third signalhaving a magnitude indicative of a third voltage level substantially midway between the first voltage level and the second voltage level; adifferential amplifier coupled to the input/output pad and coupled toreceive the third signal, a first power supply voltage, and a secondpower supply voltage, wherein the differential amplifier is configuredto amplify a difference between the voltage magnitude of the inputsignal and the third voltage level in order to produce an output signalwhich alternates between the first and second power supply voltages; andcircuitry coupled to receive the output signal and configured to performa function dependent upon the output signal.
 18. The circuit as recitedin claim 17, wherein the circuit is an integrated circuit, and whereinthe input/output pad, the input buffer and the circuitry are formed uponand within a single monolithic semiconductor substrate.
 19. The circuitas recited in claim 17, wherein the input signal is an input clocksignal, and the output signal is an output clock signal.
 20. The circuitas recited in claim 19, wherein the circuitry is synchronous dynamicrandom access memory (SDRAM) circuitry, and wherein the SDRAM circuitryuses the output clock signal to synchronize internal operations.
 21. Amethod for signal buffering, comprising: receiving an input signal,wherein a voltage magnitude of the input signal alternates between afirst voltage level and a second voltage level; producing a first signalhaving a magnitude indicative of the first voltage level of the inputsignal; producing a second signal having a magnitude indicative of thesecond voltage level of the input signal; using the first and secondsignals to produce a third signal having a magnitude indicative of athird voltage level substantially mid way between the first voltagelevel and the second voltage level of the input signal; amplifying adifference between the voltage magnitude of the input signal and thethird voltage level to produce an output signal which alternates betweena first power supply voltage and a second power supply voltage.